The significantly longer charging times of electric vehicles (EVs) compared to gasoline vehicles pose a major obstacle to widespread EV adoption. Current fast charging capabilities in EVs often require over 30 minutes to reach 80% state of charge (SOC). To achieve a refueling experience comparable to gasoline vehicles, the US Department of Energy (US DOE) has established XFC goals: <15 min charge time to 80% SOC, >180 Wh/kg discharge specific energy, and <20% capacity loss over 500 XFC cycles. Existing commercial high-energy-density LIBs, with graphite negative electrodes and transition-metal oxide positive electrodes, struggle to meet these long-term XFC cycle life requirements. Reducing charge time to 15 min necessitates a 6C charge rate during the constant-current stage of CCCV charging, which can cause lithium plating on graphite negative electrodes and lead to substantial capacity fade. Mitigating lithium plating, requiring faster ion transport and kinetics in LIBs, is a crucial challenge. Research efforts to develop XFC-capable LIBs focus on new electrolytes, electrode materials, charge protocols, and heating strategies. Heating strategies, particularly, show promise for near-term XFC implementation in existing high-energy-density LIBs. This study explores a novel thermal management approach to address these challenges.

Existing research on fast-charging LIBs broadly focuses on four areas: electrolyte innovation, development of new electrode materials, optimization of charging protocols, and the implementation of heating strategies. Heating strategies have shown the most immediate promise for improving the charge rate of existing high energy density batteries. Two main heating strategies have emerged. The first approach is system-level control using battery thermal management systems (BTMSs) that modulate the coolant temperature and flow rate. However, this approach is limited by the low cell-to-pack ratio in practical battery packs, leading to significant heat dissipation to the pack and limiting temperature rise. The second strategy involves cell-level temperature control using embedded heaters and thermal insulation. While enabling higher C-rates, this method suffers from high average battery temperatures during operation and rest, negatively impacting overall performance and lifetime, and also creates compatibility and safety concerns. This study aims to address the limitations of existing strategies by proposing a novel thermally modulated charging protocol.

This research combines computational modeling and experimental validation to demonstrate a new thermal management strategy for extreme fast charging (XFC) of commercial lithium-ion batteries (LIBs). A lumped thermal model was initially developed to analyze the transient battery temperature, considering factors like heat generation, surface area, temperature, mass, and heat capacity. The model considers the tunable thermal conductance between the battery and coolant, enabling the simulation of different thermal protocols. Two existing approaches were reviewed, namely coolant modulation (CM) and thermal insulation of the battery cell. Both these approaches were simulated and analyzed, followed by a novel strategy incorporating active thermal switching (ATS). An electrochemical-thermal (ECT) model was developed in COMSOL Multiphysics 5.6 using a Newman pseudo 2D electrochemical model coupled with a 3D transient heat transfer model. This ECT model was validated experimentally using three-electrode cells. The ECT model was used to design a thermally modulated charging protocol (TMCP) that combines active thermal switching (ATS) with coolant modulation (CM). The TMCP was optimized to achieve the desired balance between sufficient temperature elevation for fast kinetics during charging and rapid cooling after charging to mitigate side reactions. To validate the TMCP, both numerical simulation and experimental analysis were performed. In the experiments, commercial 5-Ah C||LCO LIBs were subjected to 6C1C cycling tests (6C charge and 1C discharge) under various thermal protocols including cooling, coolant modulation, insulation, and the proposed TMCP. A linear actuator was used to simulate ATS experimentally, while a prototype ATS device using shape memory alloy (SMA) was developed and integrated into a BTMS for more practical implementation. Post-mortem characterization using optical microscopy, SEM, and X-ray tomography was carried out to analyze the degradation mechanisms under different thermal protocols, focusing on lithium plating and side reactions. Electrochemical impedance spectroscopy (EIS) was also employed to study the changes in SEI resistance and to explain the capacity fade. The performance of the SMA-based thermal switch was tested and compared with the results obtained using a linear actuator. Furthermore, studies were performed to investigate the applicability of the proposed approach across various Li-ion cell chemistries (including C||NMC), initial SOCs, and ambient temperatures.

The study demonstrates that the proposed TMCP significantly outperforms existing thermal protocols for XFC. By combining active thermal switching with coolant modulation, the TMCP achieves a 6C charge rate (<15 min to 80% SOC) while mitigating lithium plating and side reactions. Key findings include: 1. **Improved XFC Performance:** The TMCP consistently outperforms other approaches, consistently achieving charging times under 15 minutes, surpassing the US DOE targets for XFC. The switching ratio (hON/hOFF) of approximately 10 was deemed optimal for achieving this performance. 2. **Mitigation of Lithium Plating:** Electrochemical analysis and post-mortem characterization confirmed that the TMCP effectively mitigates lithium plating on the graphite negative electrode, a significant degradation mechanism during fast charging, as evidenced by improved coulombic efficiency. 3. **Reduced Side Reactions:** By dissipating heat efficiently after XFC, the TMCP reduces side reactions that contribute to capacity fade. This is reflected in the extended cycle life of the batteries tested under the TMCP. 4. **Extended Cycle Life:** The TMCP enables more than 500 XFC cycles at ambient temperatures above 25 °C, exceeding the US DOE target and demonstrating improved longevity over other methods. 5. **Feasible Integration:** A prototype thermal switch based on shape memory alloy (SMA) was developed, demonstrating the feasibility of integrating the TMCP into existing BTMSs with minimal impact on battery size, weight, and cost. This integration allows for switching between high and low thermal conductance to actively control the battery temperature. 6. **Broad Applicability:** The benefits of the TMCP were demonstrated across different Li-ion cell chemistries (C||LCO and C||NMC), initial SOCs, and ambient temperatures, highlighting the general applicability of the approach. 7. **Low-Temperature Performance:** The TMCP improves low-temperature performance, achieving relative discharge energy exceeding the USABC target (70%) at both C/3 and 1C discharge rates, even at -20°C. 8. **Pack-Level Scalability:** A potential pack-level design was proposed to show the scalability of the active thermal switching strategy, indicating its applicability to larger battery packs.

The findings demonstrate that the proposed TMCP effectively addresses the key challenges associated with extreme fast charging (XFC) of commercial high-energy-density LIBs. The success of this approach lies in its ability to harness the battery's self-generated heat during charging to enhance kinetics while simultaneously implementing an efficient heat dissipation strategy after charging to minimize side reactions. This strategy avoids the limitations of previous approaches such as coolant modulation and thermal insulation, by cleverly using the battery’s own heat generation, rather than relying on external heating elements that can decrease lifetime, increase cost and raise safety concerns. The development and successful testing of the SMA-based thermal switch further demonstrates the practical feasibility of implementing this TMCP in real-world applications. The findings are particularly significant because they present a relatively simple and non-intrusive solution for enhancing XFC performance, avoiding the need for extensive modifications to the existing battery manufacturing process or the development of entirely new materials. This highlights a significant advancement in the field of battery thermal management and has the potential to accelerate the adoption of EVs by addressing one of the most significant bottlenecks—charging time.

This study presents a novel thermal management strategy, the TMCP, for enabling extreme fast charging (XFC) of commercial high-energy-density LIBs. The approach utilizes active thermal switching and leverages the battery's self-generated heat to achieve fast charging while mitigating lithium plating and side reactions. The successful development and integration of an SMA-based thermal switch, along with extensive experimental validation, demonstrate the practical feasibility and potential of this technology for improving the performance and cycle life of EV batteries. Further research could focus on optimizing the design of the thermal switch for improved efficiency and cost-effectiveness, as well as exploring its integration with other BTMS technologies. This study provides a viable pathway for addressing the XFC challenge and accelerating the widespread adoption of EVs.

While the study demonstrates significant improvements in XFC performance, there are limitations to consider. The study primarily focused on specific commercial Li-ion cell chemistries and configurations. The applicability of the TMCP to other cell chemistries and battery pack designs requires further investigation and validation. Additionally, the long-term performance of the SMA-based thermal switch over an extended number of cycles needs more detailed evaluation. Finally, the cost-effectiveness of this approach at a large scale still needs to be fully assessed.